Paper of improved strength



14 Tm. n

April 24, 1962 J. H. BEYl-:R l-:TAL 3,031,371

PAPER 0F IMPROVED STRENGTH' Filed June 16, 1959 INVENTORS Jfeyer JCara//za Lf?. Par/5.9' 3J United States Patent O 3,031,371 PAPER F IMPRVED STRENGTH .lohn H. Beyer, Memphis, rienn., John H. Foreman, Cincinnati, Chio, and Lawrence R. Parks, Memphis, Tenn., assignors to The Buckeye Cellulose Corporation, Cincinnati, Chio, a corporation of Ohio Filed June 16, 1959, Ser. No. 820,751

2 Claims. (Cl. 162-146) This invention relates to improvements in the manufacture of iine papers from furnishes containing cotton libers and more particularly is concerned with the provision of novel compositions containing the shorter fibers ofthe cotton plant, namely first-cnt and second-cut linters, which contribute superior strength and conversion characteristics to papers formed from furnishes comprised entirely of the aforementioned compositions. In addition, the invention provides for the preparation of hydroxyethyl cellulose according to certain definite process conditions which markedly and unexpectedly improve the ability of cotton linters blends of modified and unmodiiied bers to be processed into ne papers such as the bond and ledger types which heretofore have required a predominant proportion of rag stock in their furnishes in order to develop the requisite strength characteristics.

ln recent years the steadily increasing use of synthetic tibers in textiles and in fabrics which in prioryears had been composed entirely of staple cotton bers has posed a serious economic and supply problem to the manufacturers of line papers. Not only has the available supply of cotton rag stock such as shirt clippings been high in price and insufficient to meet the paper manufacturers quantity requirements, but there has been the ever present likelihood of carrying thermoplastic and other synthetic bers and deleterious substances into the papermaking process together with the rag stock to cause translucent spots and other quality defects intolerable in line papers. Due to the serious nature of the problem and the apparent long term trend toward the ever increasing use of fabrics either consisting of a blend of synthetic with natural fibers or of natural fibers subjected to a variety of surface treatments, the paper manufacturers have carried on an intensive effort to develop sources of fibers suitable for their needs without deleterious admixture of unwanted ibers and substances.

lt is natural that the paper industry should have considered the linters bleaching industry as a possible source of cotton ber for their use since this source represented a large supply of pure cotton bers in a related industry, Abut certain formidable problems which are the concern of this invention had to be met before this was possible. Stated in their bare essence these diiiiculties all revolved around the problem of processing short linters fibers, averaging 2 to 5 mm. in length, so that they would develop the strength characteristics of the longer rag stock fibers, which exceed one-half inch in length.

ln general, applicants solution to this problem, as indicated above, embodies the use of hydroxyethyl cellulose prepared from cotton linters under especially controlled conditions to produce a product having unique binding properties for the unmodified cotton linters with which it is combined in providing the furnish.

ln order to appreciate the full significance 0f the contribution to the art of papermaking by the invention disclosed herein and to properly diiferentiate it from the prior art, it is necessary to review the use made heretofore in the papermaking industry of hydroxyethyl modified fibers and to understand the change in liber properties effected by increasing levels of hydroxyethyl modification.

It is well known to prepare alkali cellulose by steeping cellulose with a sodium hydroxide solution and to react the resulting alkali cellulose with ethylene oxide to produce hydroxyethyl cellulose (HEC) according to the following general formula where RceuOH represents cellulose:

Likewise the treatment of cellulose with ethylene chlorohydrin to produce hydroxyethylcellulose is a wellknown chemical reaction.

The resulting hydroxyethyl cellulose, depending on the method and conditions .of preparation, may be roughly classified by its solubility in water and solutions containing 6 fto `10 percent of sodium hydroxide into one of three groups, (1) water insoluble alkali insoluble, (2) water insoluble alkali soluble and (3) water soluble, the listing being given in order of increased modification of the cellulose molecule. It is known that the group to which a particular sample of hydroxyethyl cellulose will belong is dependent to a major degree on the degree of substitution of its available hydroxyl groups with the hydroxyethyl group and to a minor degree on the physical treatment accorded the alkali cellulose during the modication reaction.

Thus HEChaving a degree of substitution from .01- 0.2 is generally insoluble in both water and weak alkali solutions, HEC having a degree of substitution from 0.2 to 0.4 is alkali soluble and water insoluble and HEC having a degree of substitution from 0.4 to the upper limit of three hydroxyl groups available for substitution is Water soluble. As the degree of substitution increases toward the upper limit the individual cellulose iibers of the product HEC become increasingly swollen, and nally lose their `brous character entirely in the water soluble range of substitution.

It is generally known to incorporate hydroxyethyl cellulose from all three of the substituent groups into paper furnishesv for the purpose of decreasing beating time to reach a given freeness, or relative drainage speed on a wire screen, in paper stock preparation. HEC has also been utilized for the purpose of -generally increasing strength characteristics of the finished paper such as evidenced by the testing procedures `generally in use in the paper industry for the measurement of the bursting, tensile and fold values of paper. HEC has also been incorporated in paper as a sizing material. However, the present invention is Vnot concerned with these examples of prior art but with the discovery that the steeping condition whereby the intermediate alkali cellulose is prepared is a controlling factor in the ultimate strength of papers formed from blends containing the HEC pre-V pared therefrom. l

Heretofore the improvement in paper stock preparation or final paper strength characteristics engendered by the incorporation of HEC into a paper stock had been thought to he roughly dependent on the amount of HEC addition, a marked rapid improvement resulting when the amount added is small and extending upward to a limit past which further HEC addition produced no gain in paper strength properties.

It has presently been discovered, however, that these assumptions based on the prior art are incorrect and that,

as more fully set forth hereinafter, a certain HEC pre- Y pared from Short bered cotton linters within critical relationships of process conditions, particularly as regards the caustic strength and temperature relationship involved in the processing of the intermediate alkali cellulose will, when incorporated with unmodified cotton linters, enable the attainment of strength characteristics in papers formed therefrom which are markedly superior to other HEC incorporations.

Y YIt is therefore an object of this invention to provide cotton linter bers which are modified by substitution under critical process conditions so that they contribute strength characteristics to paper which are markedly superior to strength improvements engendered by incorporating HEC prepared at other processing conditions.

It is a further object of the invention to provide a novel composition of hydroxyethyl modified bleached cotton linters with unmodified bleached cotton linters which will serve as a papermaking stock for the production of all cotton linter fiber papers. These papers have strength characteristics enabling them to replace rag stock papers and the strength characteristics of these papers are markedly superior to papers formed from blends in which the HEC fiber portion was processed under other conditions.

It is a still further object of this invention to provide a blend of hydroxyethyl modified bleached cotton linters with unmodified bleached cotton linters which blend is suitable for substitution as a major portion of the rag stock requirement inthe furnish of fine rag stock papers.

Further objects and features of the invention will be apparent from the following description of the preferred practice thereof, and from the accompanying drawing in which FIGURES 1, 2 and 3 present graphical illustrations of the data tabulated in the examples, illustrating the unexpected and markedly superior strength characteristics induced in all cotton linter fiber papers by addition of the particular hydroxyethylated short cotton fibers described herein.

The results plotted in the drawing as further explained hereinafter in Example I were obtained by beating blends composed of 20 percent second-cut linters HEC prepared from alkali cellulose steeped at varying caustic concentrations and 80 percent unmodified first-cut linters to a Canadian Standard freeness of 200 cubic centimeters as determined by TAPPI standard method T227m-5S, in an ordinary Noble & Wood 5 pound cycle beater with brass tackle. This beater is widely used in the paper industry to standardize physical tests in the laboratory since the Mullen and MIT fold of paper will vary with the freeness developed by beating the fibers of which it is composed. The blend was then made into sheets of paper for testing by TAPPI standard method T205m-58 with the exception that 75 gram per square meter sheets were produced instead of 60 gram sheets. The prepared and conditioned sheets were then tested according to TAPPI standard methods to obtain the values for Mullen and MIT fold.

In the production of bleached cotton linters, the linters are first separated from extraneous dirt, metallic matter, and field debris by hydraulic and/ or mechanical means. Next, to aid in the removal of oils and waxes, the linters are digested (cooked) in digesters of various types, such as the vertical stationary or spherical rotary types, using as a digestive liquor a dilute aqueous solution of sodium hydroxide generally having a concentration of three percent, or less, to which may be added small amounts of various Soaping or wetting agents, the total lamount of solution used being sufficient to supply an amount of sodium hydroxide equivalent to 5-10 percent of the weight of fibers in a digester. Digestion is accomplished by subjecting the linters together with the digestive liquor to a temperature and pressure of 50 to 125 p.s.i.g. steam over a period ranging from 3 to 8 hours. Subsequent to digestion and washing, the partially purified linters are bleached with chlorine. The bleaching agent may be added to the linters in the form of chlorine gas or sodium hypochlorite in one or more stages to bleach the product to the white color of the finished product. After washing and drying, the material is the bleached cotton linters referred to herein.

The modified cotton linter fibers of this invention are prepared by taking `as a starting material a quantity of cotton linters digested and bleached in the aforementioned manner, which is not a part of this invention, to a Cupraammonium viscosity of 400-600 seconds as determined by the A.C.S. Cupra-ammonium hydroxide method using a 2.5 gram sample. It is emphasized that linters pulp of the viscosity range mentioned, although suitable in every way for the practice of the present invention, is not a part of the invention and linter pulps of any viscosity otherwise usable as paper fibers may be utilized. The alkali cellulose is then prepared by first steeping the linters in caustic alkali solution at selected temperatures, the caustic concentration and temperature being chosen to result in complete conversion to alkali cellulose with minimum degradation of the cellulose. For example, at room temperatures of the order of 20-30 centigrade, the desired conversion is achieved by steeping for 15-35 minutes in caustic of 13.5-14.5 percent strength. Although the specific examples given as illustrative of this invention refer to conventional sheet steeping, it is not intended to limit the scope of this invention thereby and the alkali cellulose may be prepared by the well-known slurry process for converting bulk cellulose to alkali cellulose provided the processing conditions specified above are maintained as to time, temperature and caustic strength.

The steeped pulp is then drained of caustic solution and pressed so that the weight ratio of the cotton linter fibers plus the retained steeping caustic to that of the bone dry weight of the original fibers is about 3.

The resulting alkali cellulose cake is then reduced to crumb form in a suitable shredder such as a sigma blade mixer. After shredding, or crumbling, the alkali cellulose crumbs are placed in a closed reactor which is then evacuated, blanketed with nitrogen and reevacuated to eliminate the danger of explosion. Gaseous ethylene oxide is then introduced into the reactor until about 20% to about 30% by weight based on the bone dry weight of the original bleached second-cut cotton linter fibers has been added. The rate of addition is adjusted so that a maximum pressure of 25 inches of mercury is maintained within the reactor during the period of ethylene oxide addition and so that the temperature of the pulp mass does not exceed 70 degrees centigrade. The modified linter fibers are allowed to remain within the sealed reactor for 2O to 30 minutes following the period of ethylene oxide addition.

The modified linter fiber product of the reaction is then discharged from the reactor and repulped with w-ater to a consistency of between 2% and 5% and suliicient mineral acid, such as 10% sulphuric acid, is added to neutralize the sodium hydroxide carried with the modified fibers and so that the pH of the fiber slurry is adjusted to between 5 and 8.

As hereinbefore stated it is known to incorporate hydoxyethyl cellulose of various degrees of substitution and prepared by any of the reactions of the prior art into paper furnishes. The present invention is therefore not concerned with the general scheme of HEC production but with the discovery and reduction to practice of a relationship in the production of alkali cellulose for subsequent use in HEC paper furnishes which greatly enhances the strength of papers produced from blends of modified and unmodified cotton linters. Accordingly it is found that alkali cellulose prepared by treating cotton linters fibers with 13.5 to 14.5 percent aqueous sodium hydroxide solution at 25 centigrade and thereafter pressing the treated cellulose to a press weight ratio of about 3 will result in the unexpected strength increases in paper strength attributable to the practice of this invention when the resulting -alkali cellulose is converted to HEC and used in ya blend with unmodified cotton linter fibers in a paper furnish. Although the 25 centigrade 13.5 to 14.5 percent caustic steep is the presently preferred mode of operation, it is not intended to thus limit the invention and other operable caustic strength-temperature relationships are known to exist.

Sufiicient unmodified bleached cotton linters to result in a blend of 10 to 30 percent modified fibers and 70 to percent unmodified fibers is added to the sluiry while water is added to maintain the pumpable consistency of the slurry.

` Subsequent to this operation the slurry may be pumped directly to stock preparation equipment such as beaters or jor-dans and then to the paper machine for sheeting. Alternatively it may be dewatered and dried for handling reasons and supplied to a paper manufacturer in either bulk or sheet form as a suitable stock for an all cotton fiber paper.

The preferred mode of practicing the invention is described more particularly in the following examples, but it will be understood that limitation of `the scope of the invention is not thereby intended.

EXAMPLE I A 135 pound portion of 500 second cnpnammonium viscosity (based on the standard A.C.S. determination for viscosity using -a 2.5 gram sample) second-cut cotton linter pulp sheets were placed in 'a vertical position in a viscose steeping press. These sheets were 19.5" X 21" x .045" with the sheets cut with their long dimensions parallel to the grain or in the machine direction. An aqueous 14% caustic solution was introduced at the bottom of the press ata rate which minimized floating and wicking, i.e. at ya rate so that the rise in caustic level was approximately equal to the rise of the caustic solution in the sheets induced by capilllarity. The temperature of the caustic steeping solution was maintained iat 25 degrees centigrade and the pulpsheets remained submerged for 20 minutes.

Following this period of steeping in 14% caustic, the caustic solution was 'drained from the pulp sheets and the ram of the steeping press was advanced fto press fthe sheets to a cellulose content of 331/3 i.e. to a press weight ratio of 3.

The resulting alkali cellulose was shredded in a sigma blade type shredder and immediately charged into a reactor capable of being sealed from the atmosphere, which in this instance was a viscose type barette.

To safeguard against the explosive hazards of ethylene oxide reactions, the reactor was evacuated to a pressure of 25 inches of mercury whereupon nitrogen was introduced to bring the reactor up to latmospheric pressure. After this flushing precaution the reactor was sealed and again evacuated.

Gaseous ethylene oxide was added to the charged alkali cotton linters cellulose at a rate whereby the reaction maintained a pressure of 25 inches of mercury within the barette reactor and the temperature of the pulp mass did not exceed 70 degrees centigrade until the amount of ethylene oxide charged W-as equal to 25% of the weight of the original bone dry weight of the cotton linters placed in the steeping press. The reacted llinters were allowed to remain in the reactor for a period of 20 minutes fo-llowing the addition of the prescribed amount of ethylene oxide.

The hydroxyethyl modified second-cut cotton linters were then removed from the reactor and blended with unmodified bleached iirst-cut cotton linters in water to give a 20% blend of the modified linters together with 80% of the unmodified linters, the percentage being calculated on la cellulose basis neglecting any increase in weight of the modified fibers due to the reaction with ethylene oxide.

The consistency of the slurry was adjusted to 3% of pulp in water `and sulphulic acid was added to neutralize the sodium hydroxide introduced lwith the reacted liber and ladjust the pH of the slurry to 7.

The resulting blend was then jordaned to a Canadian Standard freeness of 750 to 800 cubic centimeters in order to decrease the beating time of the blend `and to facilitate sheet formation. The Williams freeness test is widely used inthe pulp and paper industry as an empirical measurement of the rate at which a dilute suspension of pulp may be partially dewatered and is indicative of the fibrillation and/or the degree and manner of the subdivision of the individual fibers in pulp. For :testing purposes handsheets were made yaccording to TAPP=I Standard method number T205m-58 with the exception of the sheet weight which was 75 grams per square meter. Mullen was determined according to TAPPI Standard method T403m-58, tear strength according to T4l4rn49, tensile strength according to T404m-50, and MIT fold according to T423m-50-II.

In a similar fashion other experimental runs were made varying the percentage strength of the steeping caustic while holding the other variables ywithin the limits of experimental error. The conditions and results of these experimental runs are tabulated in the following Table I. The information is also presented graphically in the attached drawing to show the `superiority of blends in which the hydroxyethyl cellulose modified portion was produced under the conditions of this invention.

Table I RESULTS AT 200 CC. CANADIAN STANDARD MIT Tens Percent Steep Press Beating Mullen, Tear Fold lbs./ NaOH Temp., IWeightl Time, lbs./ Grams N urninch in Steep C. Ratio Minsq. in. Per ber Test utes Sheet D subie Strip Folds Width The graphs of the above data appearing in the drawing as FIGURES 1, 2 and 3 illustrate the minimum beating time, and sharp increase of paper strength characteristics resulting from the use in blends of the modified linters fibers of this invention in comparison with blends utilizing modified linter fibers prepared by other processing conditions. FIGURE 1 compares the beating times for blends to reach a Canadian Standard freeness of 200 cubic centimeters with the caustic strength at which the linters of the modified portion of the blend was steeped in order to convert them to alkali cellulose. In a similar fashion FIGURES 2 and 3 compare MIT fold and M-ullen values respectively with the concentration of steeping caustic used in preparing the modified portions of the blend.

Although the present invention is best described by the results obtained lfrom the use of the herein defined modified fiber product and blend as the sole furnish in the cotton linter paper, it is not intended to limit the scope of the invention thereby and it is pointed out that addition of the HEC and 4blends of this invention will improve the strength characteristics of papers produced from even the best rag stock furnishes. Also, it is considered that all of the possible combinations of moditied first and second-cut linters prepared according to the methods described herein together with unmodified first and second-cut linters -be within the scope of the invention. Furthermore, it has been `ascertained that 14 percent sodium hydroxide is the minimum strength aqueous sodium hydroxide solution which will effect the conversion of cotton linter cellulose to cellulose-II (alkali cellulose) at the specified temperature of -about 25 degrees centigrade.

The following table gives examples of the temperature-percentage relationships where the strength of the sodium hydroxide is the minimum to eiect the conversion of cotton linter cellulose to cellulose-11.

Temperature, degrees Percentage, sodium It is emphasized that the above enumerated relationships do not relate all of the temperature percentage steeping combinations resulting in the improved capacity of the HEC produced from alkali cellulose steeped at the conditions to impart improved strength characteristics to paper. In fact an infinite number of such relationships exist at temperatures above, between, and below those cited and HEC produced at any of the relationships as hereinbefore dened will be yfound to have superior properties as a paper additive.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. A modified bleached cotton linter product produced by steeping purified and bleached short libered cotton linters in the minimum concentration of aqueous sodium hydroxide which will convert the fibers -to alkali cellulose ata given temperature for about 15 to 35 minutes, draining the -aqueous sodium hydroxide solution from the bers, pressing said bers so that the ratio of the weight of bers together with the retained sodium hydroxide solution to the bone dry Weight of the original fibers is about 3 and reacting the so-treated bers with about 20% to about 30% by weight of ethylene oxide based on the bone dry weight of the original ibers, said modied cotton linter product contributing superior beating, bursting and `folding characteristics to paper furnishes and papers composed of all cotton bers when added thereto in amounts of about 10% to about 30% by weight of the ber `stock contained in the furnish.

2. As a new composition of matter, a blend of bleached cotton linter fibers suitable -for a paper furnish comprised of about 10% to yabout 30% by weight of bleached cotton linter bers modified by treatment with ethylene oxide comingled with about 90% to about 70% of unmodified bleached cotton linter lfibers, the hydroxyethylation of the modified por-tion of said blend including the step of steeping bleached cotton linter fibers in the minimum concentration of aqueous sodium hydroxide which will convert the fibers to alkali cellulose at a given temperature for about 15 to about 35 minutes.

References Cited in the le of this patent UNITED STATES PATENTS 1,857,100 McCormick et al. May 3, 1932 2,488,631 Kunz Nov. 22, 1949 V2,533,145 Schorger Dec. 5, 1950 2,572,039 Klug et al. Oct. 23, 1951 2,682,535 Broderick June 29, 1954 2,736,653 Erickson Feb. 28, 1956 2,833,614 Kersnar May 6, 1958 2,847,411 Mitchell et al. Aug. 12, 1958 2,916,413 Harpham Dec. 8, 1959 

2. AS A NEW COMPOSITION OF MATTER, A BLEND OF BLEACHED COTTON LINTER FIBERS SUITABLE FOR A PAPER FURNISH COMPRISED OF ABOUT 10% TO ABOUT 30% BY WEIGHT OF BLEACHED COTTON LINTER FIBERS MODIFIED BY TREATMENT WITH ETHYLENE OXIDE COMINGLED WITH ABOUT 90% TO ABOUT 70% OF UNMODIFIED BLEACHED COTTON LINTER FIBERS, THE HYDROXYETHYLATION OF THE MODIFIED PORTION OF SAID BLEND INCLUDING THE STEP OF STEEPING BLEACHED COTTON LINTER FIBERS IN THE MINIMUM CONCENTRATION OF AQUEOUS SODIUM HYDROXIDE WHICH WILL CONVERT THE FIBERS TO ALKALI CELLULOSE AT A GIVEN TEMPERATURE FOR ABOUT 15 TO ABOUT 35 MINUTES. 